There are many industrial and other applications in which it is necessary to measure and control the flow rate of fluids, particularly gases. Typically, gas flow is measured and controlled using volumetric flow devices such as turbine meters, rotometers, thermal mass flow rate control devices, or sonic gas velocity orifices.
The need for precision control is particularly acute in the semiconductor industry. Computer chip manufacturing requires exact control of various process fluids and gases, including but not limited to hydrogen, silane, helium, nitrogen, oxygen and argon. The current "state of the art" in the semiconductor industry utilizes a sophisticated gas delivery system, often referred to as a gas panel incorporating "gas sticks", which includes a mass flow controller, a pressure transducer, a filter, control valves and a pressure regulator, all connected in series. The flow control portion of these systems have high initial and maintenance costs, require frequent calibration and service to avoid inaccuracies caused by electronic drift and span, and may result in inaccurate flow rates when very high or very low flow rates are required.
In situations in which repeatability is more important than absolute accuracy, precision calibrated orifices have been used to provide a constant calibrated gas flow relative to gas supply pressure; if multiple fixed flow rates are needed, a number of orifices may be connected in parallel with each other with a switching mechanism for selecting the appropriate orifice. However, the use of such orifices is normally limited to applications that require one or more constant, non-variable gas flows. Even in fixed flow applications where their use is otherwise satisfactory, such orifices require high gas velocities which cause excessive turbulence, erosion therefore and flow instability, and are subject to plugging.
Precision porous sintered metal flow restrictors, (e.g., of the type manufactured and sold by Mott Corporation, the assignee of the present application and which have hundreds of interconnected through-pores or passages arranged both in parallel and series with each other) are also used to provide a specified down-stream flow relative to the applied upstream pressure. Such flow restrictors are less susceptible to plugging, clogging and wear than are conventional orifices, operate at relatively low flow velocities, and provide a smooth and constant down-stream flow. Like orifices, however, their use has been limited to applications that require an essentially constant and non-variable flow.
There remains a need for a system that, like a thermal mass flow controller, is capable of precisely measuring and controlling fluid flow over a range of flow rates and pressures, but that is more accurate over a wide range of flow rates, is less expensive, and that requires significantly less calibration, servicing and maintenance and is less susceptible to electronic drift and span.